Deformation and strength of engineering ceramics and single crystals

Abstract A comparative investigation has been performed of the laws of deformation and strength of engineering silicon nitride-, alumina, and zirconia-based ceramics, as well as zirconia crystals. The specimens were tested in four-point bending in air over a temperature range from ambient to 1400 °C. The materials' mechanical behaviour was analyzed by load versus deflection diagrams (deformation diagrams). It has been found that above the brittle-to-ductile transition temperature, TBD, for the ceramics studied it is possible to choose such combinations of temperature and deformation rate, which would ensure the same path of deformation diagrams, i.e. equivalent influence of those two parameters on the mechanical behaviour of ceramics is observed. The mechanism of inelastic deformation of the ceramics studied is shown to be associated with the development of two processes: viscous flow of the grain boundary phase and crack propagation to a critical size, with one of them prevailing at different load levels. In contrast to ceramics, the strength of zirconia crystals remained practically unchanged (partially-stabilized crystals) up to 1400 °C or even increased (fully-stabilized crystals). For all the crystals, static elasticity moduli were independent of the number of repeated-static loading cycles at high temperatures (their values remained constant), while for ceramics a reduction was observed. Partially-stabilized zirconia crystals, as compared to other investigated materials, exhibited practically no creep up to 1400 °C at loads close to limiting ones. This allows them to be considered as a promising engineering material for high-temperature application.

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